Process for operating a plasma arc torch

ABSTRACT

A process for operating a plasma torch having an improved start up and shut down sequence to substantially increase electrode element life. The first shut down mode is controlled such that plasma gas flow through the swirl ring and nozzle prevents the formation of an oxide layer upon the electrode. The shut down method is especially useful for torches which operate at 100 amps or greater. The inclusion of an aluminum jacket surrounding the electrode outer walls has been found to contribute to the ability to avoid formation of oxide layers on the electrode.

RELATED APPLICATIONS

This application is a Continuation-in-Part of U.S. application Ser. No.09/540,077 filed on Mar. 31, 2000 and which is an Continuation-in-Partof application Ser. No. 09/416,304 filed Oct. 12, 1999 and issued asU.S. Pat. No. 6,093,905 and application Ser. No. 09/178,207 filed Oct.23, 1998 and issued as U.S. Pat. No. 6,163,009 which are incorporatedherein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to a process for operating aplasma arc torch, and more particularly to a start up sequence and ashut down sequence that significantly extends the life of the electrodeand nozzle.

The operation of conventional plasma arc torches is well understood bythose in the art. The basic components of these torches are a body, anelectrode mounted in the body, a nozzle defining an orifice for a plasmaarc, a source of ionizable gas, and an electrical supply for producingan arc in the gas. Upon start up, an electrical current is supplied tothe electrode (generally a cathode) and the pilot arc is initiated inthe ionizable gas typically between the electrode and the nozzle, thenozzle defining an anode. Then, a conductive flow of the ionized gas isgenerated from the electrode to the work piece, wherein the work piecethen defines the anode, and a plasma arc is thus generated from theelectrode to the work piece. The ionizable gas can be non-reactive, suchas nitrogen, or reactive, such as oxygen or air.

A significant problem with conventional plasma arc torches is wear ofthe electrodes and nozzles. Typically, the electrodes include a hafniumor a zirconium insert. These materials are desired for their materialproperties, but are extremely costly and require frequent replacement.

It has been found that a significant percentage of the electrode wearand damage occurs during shut down of the torch. It is believed that oncut off of electrical current to the electrode, wear results from acomplicated interaction between molten surfaces of the electrode and thepressurized flow of the plasma gas through the nozzle. The phenomena isalso described in U.S. Pat. No. 5,070,227 which is incorporated hereinby reference.

One form of electrode wear includes the formation of an oxide such asHafnium oxide or Zirconium oxide. As set forth in the Applicant's priorU.S. Pat. No. 6, 093,905 which is incorporated herein by reference, theoxidation of the electrode is a major contributor to electrode wear andloss. Applicant's prior U.S. Patent describes a shut-down protocol for aplasma arc torch which is varied over the life of the electrode so as tobeneficially remove accumulated oxide material from the electrode.

It is also understood that the electrodes, and particularly the inserts,have a limited number of cycles or “pierces”. A “pierce” refers to thestarting up and initial cutting or piercing of the arc through a workpiece. For each pierce there is obviously a prior shut down of the torchand an associated start up sequence. Plasma torches utilizingconventional shut down methods and operating above 100 amps have anelectrode life of generally between about 400 to 800 pierces.

The industry is constantly seeking methods for improving the plasmatorches, and particularly for extending the life and improving the wearcharacteristics of the electrodes. The present invention concerns justsuch an improved method.

OBJECTS AND SUMMARY OF THE INVENTION

It is, therefore, a principal object of the present invention to providea process for operating a plasma arc torch, particularly on shut down,that significantly reduces oxidation formation of the electrode element,and loss of molten element material and thereby reduce electrode elementerosion and eliminate nozzle damage.

An additional object of the present invention is to provide a processfor shutting down conventional plasma arc torches that can be readilypracticed by conventional torches with relatively minor modifications.

Additional objects and advantages of the invention will be set forth inpart in the following description, or may be apparent from thedescription, or may be learned through practice of the invention.

In accordance with the objects and purposes of the invention, a processis provided for operating a plasma arc torch on shut down. The processoperates on the principles of substantially reducing the oxidation ofthe electrode and further eliminating loss of molten electrode materialduring the shut down sequence in order to substantially increase thelife of the electrode. It has been found by Applicants that, throughpractice of the present invention, electrode life of conventionaltorches can be extended as much as four-fold.

It is another aspect of the present invention to provide a new anduseful start up sequence which controls the gas flow relative to the arccurrent. Applicant's start up method eliminates nozzle damage, reduceselectrode erosion, and maintains a stable arc. The improved start upmethodology may use conventional gas supply and control means. The startup method has the additional advantage of reducing damage to torchcomponents caused by molten metal during piercing of the work pieceduring start up. The start up sequence allows a more rapid penetrationthrough material and thus minimizes molten metal blow back that mayotherwise damage the torch frontend components.

The invention will be described in greater detail below through use ofthe appended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic view of a plasma arc torch having thegas supply means and pathway useful for practicing the presentinvention;

FIG. 2 is a graph illustrating an embodiment of a start up sequenceaccording to the invention; and,

FIG. 3 is a graph illustrating an embodiment of a shut down sequenceaccording to the invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the presently preferredembodiments of the invention, one or more examples of which areillustrated in the drawings. Each example is provided by way ofexplanation of the invention, and not meant as a limitation of theinvention. For example, features illustrated or described as part of oneembodiment, can be used with another embodiment to yield still a furtherembodiment. It is intended that the present invention include suchmodifications and variations as come within the scope of the invention.

FIG. 1 is a simplified schematic view of a conventional plasma arctorch, similar to the FL 100 plasma arc torch provided by InnerLogic,Inc. of Charleston, S.C. It should be appreciated, however, that thepresent inventive method is not limited to any particular type of plasmaarc torch. Any manner of conventional torches may be modified by theinclusion of an aluminum sleeve which surrounds the cylindrical walls ofthe electrode element. When combined with the start up and shut downsequences of the present invention, greater electrode element life andoperating efficiencies may be obtained. For example, U.S. Pat. No.5,070,227 describes a control process applicable to a wide variety oftorches, including torches sold by HyperTherm, Inc. of Hanover, N.H. Thepresent control method is applicable to the types of torches describedin the '227 patent upon modification of the electrode with the aluminumjacket as described below. The '227 patent is incorporated herein byreference in its entirety for all purposes.

The operation of conventional torches is well understood by thoseskilled in the art and a detailed explanation thereof is not necessaryfor purposes of this disclosure. The following description is forbackground purposes relating to conventional plasma arc torches ingeneral and provides additional details of a torch apparatus and methodsof torch start up and shut down in accordance with the presentinvention.

Referring to FIG. 1, plasma arc torch 10 has a basic body, generallyindicated as 12. Body 12 includes a torch supply tube 34 defining asupply chamber 36 that is supplied with a source of pressurizedionizable gas from gas supply 24 through gas supply line 26. Remotelyactuated valves, such as solenoid valves 28A, 28B and 28C are disposedin-line between supply tube 34 and gas source 24 to vary the supply ofvarious gases to torch 10 upon actuation of the valve.

As is appreciated by those skilled in the art, the plasma gas may benon-reactive, such as nitrogen, or reactive, such as oxygen or air.Torch body 12 includes an electrode body 46, typically formed fromcopper. An electrode insert or element 50 is fitted into the lower endof electrode body 46. An aluminum sleeve 51 forms a jacket around thecylindrical walls of element 50. Element 50 is typically formed ofhafnium or zirconium, particularly when the plasma gas is a reactivegas.

An insulating body 38 generally surrounds the supply tube 34 andelectrode body 46. The cathode body 40 is disposed generally surroundingsupply tube 34 and an anode body 42 is disposed surrounding insulatingbody 38.

A nozzle 16 is disposed at the forward end of electrode body 46 anddefines an arc passageway 52 aligned with electrode element 50.

A swirl ring 44 is disposed around the electrode body 46 and has holesdefined therein to induce a swirling component to plasma gas enteringthe plasma gas chamber 14, as will be discussed in greater detail below.

A power supply 18 is provided to supply electrical current to electrodebody 46 and the electrode element 50. A negative power lead 29 is inelectrical communication with supply tube 24 and cathode body 40. In apilot arc mode, a positive power lead 22 is in electrical communicationwith anode body 42 through switch 23. Insulating body 38 electricallyisolates anode body 42 from cathode body 40. Positive power lead 22 isalso connectable to a work piece 54 that is to be cut or pierced by theplasma torch once switch 23 is opened. Power supply 18 may constituteany conventional dc power supply sufficient to provide current to thetorch at an appropriate voltage to initiate the pilot arc and thenmaintain the arc in the operational cutting mode of the torch.

In operation, plasma gas flows from source 24, through valves 28B and28C, and enters supply line 26, as generally indicated by the arrows inFIG. 1. The plasma gas flows downward in chamber 36 through orifices inswirl ring 44 before entering the lower plasma gas chamber 14. It shouldbe understood that lower plasma gas chamber 14 is in communication withthe entirety of the supply chamber 36 of supply tube 34 so that a changein pressure anywhere within the system will affect a change in pressurewithin lower plasma gas chamber 14.

In the pilot arc mode of torch 10, switch 23 is closed so that thepositive lead is connected to anode body 42. Power supply 20 providescurrent at the appropriate voltage to initiate the pilot arc betweenelectrode element 50 and nozzle 16. A desired plasma gas flow andpressure are set by the operator for initiating the pilot arc. The pilotarc is started by a spark or other means, such as a contact startingtechnique, all of which are known in the art.

The plasma gas flow during the pilot arc mode is from supply 24, throughvalves 28A, 28B and 28C, and through supply line 26 into supply chamber34, through the holes in swirl ring 44, into lower plasma chamber 14,and out through arc passageway 52 of nozzle 16. The swirling flowgenerated by swirl ring 44 is desired as a means for stabilizing the arcin all operational modes so that the arc does not impinge on and damagethe nozzle.

In order to transfer torch 10 to the cutting mode, the torch is broughtclose to workpiece 54 so that the arc transfers to the workpiece 54 asswitch 23 opens. The transferred arc now passes current from theelectrode to workpiece 54. The current is increased to a desired levelfor cutting such that a plasma arc 56 is generated which extends througharc passageway 52 to workpiece 54. The operational current levels dependon the type of torch and application desired, and typically range fromabout 20 to about 200 amps. As the operational current is increased, theplasma gas within lower plasma chamber 14 heats up and a decrease inplasma gas flow out of nozzle 16 results. In order to sustain sufficientplasma gas flow through nozzle 16 to sustain the plasma arc 56, pressureof the plasma gas being supplied must be increased with the increase ofcurrent.

The start up and shut down processes according to the present inventionwill now be described in greater detail through use of the diagrams ofFIGS. 1-3.

As already described, a critical concern with conventional plasma arctorches is the electrode life, and particularly the life of the hafniumor zirconium inserts. It is known that the start up and shut downprocess results in severe wear of the electrode elements. The presentApplicants have discovered that the electrode life can be substantiallylengthened by following an improved start up sequence. Additionally,electrode life can be further lengthened by following an improved shutdown sequence which substantially eliminates the oxidation of theelectrode during a shut down mode. Further, the shut down protocolminimizes loss of the molten electrode insert material.

Although not wishing to be bound by any particular theory, Applicantsbelieve that at least one of the factors contributing to wear on theelectrode elements is the build up of an oxide layer on the element. Ithas been found in accordance with one aspect of the present inventionthat the shut down mode of the present invention substantiallyeliminates the formation of an oxide layer.

On tests conducted on a FL 200 plasma arc torch using a conventionalstart up sequence and a no post-flow first shut down mode, the electrodelife was about 300 to 600 pierces. The same model torch was testedaccording to both the start up and shut down method of the presentinvention. The electrode life was extended to about 1,500 pierces. It isbelieved that this substantial increase in the electrode life was due tothe chemical combination of aluminum, hafnium, and other elementmaterials which may be present during the current reduction (insertcooling) that substantially eliminates oxide formation. Further, theloss of unsolidified electrode element's components which results fromthe post-flow of gases past the nozzle is further minimized by thepresent invention.

It should be appreciated by those skilled in the art that the particularnumber of pierces obtained for any one electrode will depend on theparticular type of torch, insert material, current levels, etc. The bestcombination of materials for any given application may be empiricallydetermined or estimated.

FIG. 2 represents a conceptual timing phase of one embodiment of a startup sequence according to the invention. The start up of the plasma arctorch involves the interaction of the plasma arc current, the plasma gasflow rate through the nozzle, and the gas pressure of the plasma gaswithin plasma gas chamber 14, where chamber 14 is defined generallybetween the swirl ring 44 and the nozzle 16. The left hand side of thegraph of FIG. 2 represents a start up sequence of the present invention.The end values on the far right hand side of the graph of FIG. 2represents the operational steady state values Io, Fo, and Po of therespective current and gas flow parameters.

In reference to FIG. 2, during the start up sequence beginning at t0, apre-flow pressure source is provided through valve 28A until time t3. Att3, valve 28B is turned on and immediately blocks the pre-flow gas from28A. At the t3 point, the operating nozzle flow and torch pressure flowsare introduced. The present invention introduces an increase in the arccurrent upslope rate from t4 to t5. The upslope increase coincides withthe increase beginning at t3 in the plasma gas flow rate through thenozzle. The increase in the arc current slope rate from t4 to t5, inconjunction with a similar increase in the plasma gas flow rate throughthe nozzle, has been found to decrease erosion of the electrode duringstart up. When gas flow is too low relative to the current, nozzledamage may occur to the nozzle components. When gas flow is too highrelative to the current, erosion of the electrode is increased.

In the present invention, the arc current is increasing between intervalt2 to t5. The rate (slope) of the arc current is further increased att4. Increasing the arc current is normally associated with a decrease inthe nozzle flow as the arc current reduces the nozzle flow. Thisincrease in the upslope rate of the arc current occurs immediately afterthe introduction at t3 of the operating nozzle flow and torch pressure.The upslope increase in arc current brings about a concomitant decreasein nozzle flow which would otherwise occur, thereby avoiding a nozzleflow gas overshoot. As a result, the current upslope rate increase istimed in conjunction with the interval during which nozzle gas flow andpressure are also rapidly increasing. As the current increases, the rateof nozzle flow increase is regulated, decreasing damage to the electrodeelement.

In the example set forth in FIG. 2, the value at interval t4 for the arccurrent is 87 amps at 182 msec. It has been found in accordance withthis invention that the sharp increase in arc current slope from t4 tot5 reduces damage to the torch during start up. The improvement relatesin part to the reduction in molten metal blowback during the piercingoperation. The sharp current increase as reflected in the rate of risehas been found to more quickly penetrate the substrate and minimizedamage of the torch components from the molten substrate.

It has additionally been found that the torch electrode life isprolonged by the further use of an improved torch shut down sequence.The shut down sequence makes use of the gas control components as seenand described in reference to FIG. 1. During the steady state operationof the torch, gas control valve 28A is closed stopping the pre-flow gasand commencing plasma gas flow through valves 28B (ports 1 and 2) and28C (ports 2 and 3) at a pressure of about 65 psi. A shut down sequenceis initiated at interval t0 which coincides with a de-energization ofvalve 28B and thereby closes the respective port 1. The de-energizationof valve 28B thereby ceases the flow of plasma gas to the torch andresults in an immediate decrease of pressure and flow rate through thenozzle as seen in reference to FIG. 3.

As further set forth in FIG. 3, between intervals t0 and t1, the arccurrent is decreased from 200 amps to 115 amps, reducing by aboutfour-fold the heat load on the electrode. Simultaneously, the plasma arcdiameter begins to decrease, thereby exposing the molten electrodeelement material to the plasma gas. In accordance with this invention,it has been found that the inclusion of the aluminum jacket 51surrounding the electrode has been found to largely eliminate theformation of oxides on the electrode which would otherwise occur. Thealuminum jacket can have a wall thickness of between 0.002 and 0.02inches, and preferably about 0.004 inches.

While not wishing to be limited by theory, it is believed that thealuminum jacket 51 may actually melt and combine with the electrodeelement during operation of the torch. For instance, a hafnium electrodenormally has a uniform, dark gray coloration. Under operating conditionswhen oxides form, a white marbling effect occurs which is attributed tothe formation of an oxide. Upon inclusion of the aluminum jacket inassociation with the electrode element, the resulting electrode materialassumes a grayish color which is distinctive from the initial hafniummaterial and distinctly different in appearance from previously observedoxides.

The electrode element/aluminum electrode combination is still conductiveand retains the useful properties of the hafnium and has the additionalattributes of preventing the loss of efficiency associated with oxideformation on the electrode element. It is possible that the aluminumjacket eliminates the formation of oxidation products or, alternatively,the aluminum combines with the oxide to form yet another product whichavoids the detrimental effects of the oxide. Irrespective of which, ifany, of the above processes is occurring, the combination of thealuminum jacket with the electrode element achieves significantadvantages in extending the electrode life.

In accordance with this invention, it is believed than an improvedelectrode and electrode element may be provided for plasma arc torches.Such an electrode may be provided by an alloy of aluminum and hafnium oran alloy of aluminum and zirconium. To the extent the present inventionproduces an electrode which comprises a molten mixture of aluminum andhafnium, similar electrodes may be prepared and marketed for use inplasma arc torches. The relative amounts of aluminum or aluminumcontaining metals in combination with the zirconium or hafnium materialmay be readily determined by routine experimentation in which theresulting electrode life is monitored for various combinations ofmaterial.

At interval t1, valve 28C is also energized which connects thepressurized post-flow, through orifice 53 of a flow restrictor 55 to theplasma torch inlet. The diameter or orifice 53 should be selected incombination with the gas flow equipment and selected pressures so as toprovide the desired post-flow pressure value. One having ordinary skillin the art is able to provide an appropriate flow restrictor and orificetherethrough so as to achieve the desired post-flow gas pressure neededfor any individual torch.

The post-flow pressure is maintained at a lesser pressure value than theplasma flow pressure. In the example set forth in FIG. 3, a post-flowoperating pressure of about 6 pounds per square inch has been founduseful. The 6 pounds of pressure is less than the pressure associatedwith the operating nozzle flow. As a result, the pressurized post-flowserves as a vent-like sink and helps maintain the correspondence betweengas flow and arc current during the shut down sequence. By interval t2,the post-flow pressure is sufficiently greater than the plasma gaspressure such that the post-flow direction is now toward the rear of thetorch and provides for nozzle flow rate of about 30% of the cutting flowrate at the time the arc current is shut off.

At interval t2, the arc current is shut off. The t2 interval should beselected at a point where the current is still sufficient to maintain atransferred arc. At the same time, the swirl flow should be high enoughto maintain a stable arc while the nozzle pressure and post-flowpressures are sufficiently low to avoid a damaging level of gas flowovershoot when the arc is extinguished. It has been found useful tomaintain a post-flow gas flow rate at interval t2 which is about 30% ofthe cutting plasma flow rate. As seen in reference to FIG. 2, thepost-flow continues past interval t2 when the arc is extinguished. Theresulting decrease in gas flow at the time the arc extinguishes has beenfound to greatly extend the electrode life.

The gas flow apparatus and process set forth above provides for a smoothtransition of gas flow and gas pressures when the arc is extinguished.The gas flow apparatus serves as both a pressurized sink to reduce thetorch plasma pressure and also provides a gas supply source prior tointerval t2. After the current is reduced to 115 amps, the post-flowvalve is actuated and pressure within the torch is allowed to equalizeto the post-flow pressure. This results in the rapid decrease in is theswirling and plasma flow gas pressures. After the pressures haveequilibrated, the post-flow no longer operates as a sink but becomes asource, providing the swirl and post-flow pressures. The change overfrom a gas sink to a gas source occurs about mid-way between intervalst1 and t2. The change over eliminates any pressure disturbances normallyassociated with a valve controlled gas flow.

The above shut down protocol and sequence has been found particularlyuseful for plasma arc torches operating at currents of 100 amps orgreater. In particular, the present invention has been found to maintaina good correspondence between gas flow and arc current during both startup and shut down sequences. As such, the formation of oxides on theelectrode is significantly reduced. During the shut down protocols, theerosion of the electrode and the formation of oxides on the electrodeare reduced by maintaining an adequate swirling gas flow at the timecurrent is cut off. Further, the post-flow gas characteristics aredesigned to prevent flow overshoot at current cut off, therebyminimizing damage to the electrode. In addition, the shut down protocolreduces the electrode temperature prior to arc current cut off, againminimizing the electrode erosion and electrode oxidation. Additionalimprovement is realized by use of the sink-source post-flowcharacteristics of the invention which minimizes the shut downprocessing time. By shortening the time required for the shut downprotocol, erosion of the electrode element caused by premature arccurrent loss is eliminated.

The present invention describes a shut down protocol in which thecurrent and the gas flow rate are sloped down in a continuous andcorresponding manner. However, it is understood that similarimprovements may be obtained by stepping down the current to a lowerlevel such as 20 amps and running the torch for a brief interval whilethe torch cools down. Following cool down, the arc current may be shutoff.

It should be appreciated by those skilled in the art that there are anumber of variations and modifications that may be made in the shut downprocess according to the present invention. For example, a number ofpost flow and no post-flow shutdown methods may be utilized as the firstand second shutdown modes according to the invention. It is intendedthat the present invention include such modifications and variations ascome within the scope and spirit of the appended claims and theirequivalents.

What is claimed is:
 1. A process for starting a plasma torch, the plasmatorch having a plasma gas chamber supplied with a plasma gas, anelectrode supplied with current for generating a plasma cutting arc byionization of the plasma gas, and a nozzle disposed in front of theelectrode through which the plasma cutting arc extends to a workpiece,in an operational cutting mode of the torch the plasma gas within theplasma gas chamber having a swirl component imparted thereto by a swirlring, said start up process comprising: increasing an arc current at afirst upslope rate for a first time interval (t2 to t4); introducing anoperational nozzle gas flow and pressure at t3; increasing the upsloperate of the arc current increase during a second time interval (t4 tot5), said upslope rate increase thereby regulating the nozzle flow toprevent the nozzle flow from exceeding an operational flow rate Foduring start up; and, establishing an operational arc current and anoperational nozzle flow rate.
 2. A process of adjusting a pressurewithin a plasma gas chamber of a plasma arc torch comprising: providinga plasma arc torch having a plasma gas pressure within a plasma gaschamber; reducing to zero the plasma gas supply source, therebyinitiating a pressure decrease within the chamber; supplying a post-flowpressure source in communication with said plasma gas chamber, saidpost-flow pressure source having a pressure greater than the atmosphere,but less than a value of said decreasing pressure within the chamber;and, equilibrating the pressure within said chamber with said pressureof said post-flow pressure source.
 3. The process according to claim 2wherein said post-flow pressure source is supplied at a pressure betweenabout 2 psi to about 10 psi.
 4. The process according to claim 2 whereinsaid post-flow pressure source is supplied at a pressure of about 6 psi.5. The process according to claim 2 wherein said supply of post-flow gascontinues to flow past an electrode of the plasma arc torch.
 6. Aprocess for operating a plasma torch on shut down, the plasma torchhaving a plasma gas chamber supplied with a plasma gas, an electrodesupplied with current for generating a plasma cutting arc, and a nozzledisposed adjacent the electrode through which the plasma cutting arcextends to a workpiece, in an operational cutting mode of the torch theplasma gas within the plasma gas chamber having a swirl componentimparted thereto, said process comprising: decreasing the current to theelectrode from I_(o) to a lesser intermediate value during a timeinterval from t0 to t1, said current being reduced at a first rate;decreasing proportionately a nozzle flow rate along with said decreasein arc current from t0 to t1; increasing the rate of current reductionfrom a time interval t1 to t2; supplying a post-flow gas source, saidpost-flow gas source in communication with said plasma gas chamber, saidpost-flow gas source having a pressure less than an initial pressurewithin said plasma gas chamber and thereby providing a pressure sink forplasma gas contained within the plasma gas chamber; establishing anequilibrium between said post-flow pressure and a pressure within saidtorch chamber; and, following said equilibrium, said post-flow pressureproviding a source for said nozzle flow gas, said post-flow gascontinuing past the electrode subsequent to the step of shutting off thecurrent to the electrode.
 7. A process for starting a plasma torch, theplasma torch having a plasma gas chamber supplied with a plasma gas, anelectrode supplied with current for generating a plasma cutting arc, anda nozzle disposed adjacent the electrode through which the plasmacutting arc extends to a workpiece, in an operational cutting mode ofthe torch the plasma gas within the plasma gas chamber having a swirlplasma gas within the plasma chamber, said start up process comprisingthe steps of: increasing a current at a first upslope rate for a firsttime interval (t2 to t4); introducing an operational nozzle flow andpressure at t3; increasing the upslope rate of the current increaseduring a second time interval (t4 to t5), said upslope rate increasethereby regulating the nozzle flow to prevent the nozzle flow fromexceeding an operational flow rate Fo during start up; and, establishingan operational arc current and an operational nozzle flow rate.
 8. Aprocess of adjusting a pressure within a plasma gas chamber of a plasmaarc torch comprising the steps of: providing a plasma arc torch having aplasma gas pressure within a plasma gas chamber; reducing to zero theplasma gas supply, thereby initiating a pressure decrease within thechamber; supplying a post-flow pressure in communication with saidplasma gas chamber, said post-flow pressure having a pressure greaterthan the atmosphere, but less than a value of said decreasing pressurewithin the chamber; and, equilibrating the pressure within said chamberwith said pressure of said post-flow pressure.
 9. The process accordingto claim 8 wherein said post-flow pressure source is supplied at apressure between about 2 psi to about 10 psi.
 10. The process accordingto claim 8 wherein said post-flow pressure source is supplied at apressure of about 6 psi.
 11. The process according to claim 8 whereinsaid supply of post-flow gas continues to flow past an electrode of theplasma arc torch.
 12. The process according to claim 8 wherein saidpost-flow gas provides a source for said nozzle flow gas.
 13. Theprocess according to claim 8 wherein said post-flow gas rate at t2 isabout 30 percent of said operating flow rate Fo.